Energy and protein requirements of Holstein × Gyr crossbred heifers

Nutrient requirements in cattle are dependent on physiological stage, breed and environmental conditions. In Holstein × Gyr crossbred dairy heifers, the lack of data remains a limiting factor for estimating energy and protein requirements. Thus, we aimed to estimate the energy and protein requirements of Holstein × Gyr crossbred heifers raised under tropical conditions. Twenty-two crossbred (½ Holstein × ½ Gyr) heifers with an average initial BW of 102.2 ± 3.4 kg and 3 to 4 months of age were used. To estimate requirements, the comparative slaughter technique was used: four animals were assigned to the reference group, slaughtered at the beginning of the experiment to estimate the initial empty BW (EBW) and composition of the animals that remained in the experiment. The remaining animals were randomized into three treatments based on targeted rates of BW gain: high (1.0 kg/day), low (0.5 kg/day) and close to maintenance (0.1 kg/day). At the end of the experiment, all animals were slaughtered to determine EBW, empty body gain (EBG) and body energy and protein contents. The linear regression parameters were estimated using PROC MIXED of SAS (version 9.4). Estimates of the parameters of non-linear regressions were adjusted through PROC NLIN of SAS using the Gauss–Newton method for parameter fit. The net requirements of energy for maintenance (NE_m) and metabolizable energy for maintenance (ME_m) were 0.303 and 0.469 MJ/EBW^0.75 per day, respectively. The efficiency of use of ME_m was 64.5%. The estimated equation to predict the net energy requirement for gain (NE_g) was: NE_g (MJ/day) = 0.299 × EBW^0.75 × EBG^0.601. The efficiency of use of ME for gain (k_g) was 30.7%. The requirement of metabolizable protein for maintenance was 3.52 g/EBW^0.75 per day. The equation to predict net protein requirement for gain (NP_g) was: NP_g (g/day) = 243.65 × EBW^?0.091 × EBG. The efficiency of use of metabolizable protein for gain (k) was 50.8%. We observed noteworthy differences when comparing to ME and protein requirements of Holstein × Gyr crossbred heifers with other systems. In addition, we also observed differences in estimates for NE_m, NE_g, NP_g, k_g and k. Therefore, we propose that the equations generated in the present study should be used to estimate energy and protein requirements for Holstein × Gyr crossbred dairy heifers raised in tropical conditions in the post-weaning phase up to 185 kg of BW.